Computer Security Secure Communication – Cryptography Acc661 Auditing of Advanced Accounting Information Systems (Spring 2002) Secure Communication – Cryptography Guest Lecture by Dr. Sanjay Goel State University of New York at Albany January 28, 2002

Secure Communication Well established needs for secure communication War time communication Business transactions Illicit Love Affairs Requirements of secure communication Secrecy Only intended receiver understands the message Authentication Sender and receiver need to confirm each others identity Message Integrity Ensure that their communication has not been altered, either maliciously or by accident during transmission

Cryptography Cryptography is the science of secret, or hidden writing It has two main Components: Encryption Practice of hiding messages so that they can not be read by anyone other than the intended recipient Authentication Ensuring that users of data/resources are the persons they claim to be and that a message has not been surreptitiously altered

Encryption - Cipher Cipher is a method for encrypting messaged Encryption algorithms are standardized & published The key which is an input to the algorithm is secret Key is a string of numbers or characters If same key is used for encryption & decryption the algorithm is called symmetric If different keys are used for encryption & decryption the algorithm is called asymmetric Plain Text Encryption Algorithm Cipher Text Decryption Algorithm Plain Text Key A Key B 1. Must have done this as an assignment last year.

Encryption - Symmetric Algorithms Algorithms in which the key for encryption and decryption are the same Cryptography is the Example: Caesar Cipher Types: Block Ciphers Encrypt data one block at a time (typically 64 bits, or 128 bits) Used for a single message Stream Ciphers Encrypt data one bit or one byte at a time Used if data is a constant stream of information

Symmetric Encryption – Key Strength Strength of algorithm is determined by the size of the key The longer the key the more difficult it is to crack Key length is expressed in bits Typical key sizes vary between 48bits and 448 bits Set of possible keys for a cipher is called key space For 40-bit key there are 240 possible keys For 128-bit key there are 2128 possible keys Each additional bit added to the key length doubles the security To crack the key the hacker has to use brute-force (i.e. try all the possible keys till a key that works is found) Super Computer can crack a 56-bit key in 24 hours It will take 272 times longer to crack a 128-bit key (Longer than the age of the universe) 1. Must have done this as an assignment last year.

Symmetric Algorithms – Caesar Cipher Caesar Cipher is a method in which each letter in the alphabet is rotated by three letters as shown A B C D E F G H I J K L M N O P Q R S T U V W X Y Z D E F G H I J K L M N O P Q R S T U V W X Y Z A B C Let us try to encrypt the message Attack at Dawn Assignment: Each student will exchange a secret message with his/her closest neighbor about some other person in the class and the neighbor will decipher it. 1. Must have done this as an assignment last year.

Symmetric Algorithms - Caesar Cipher Encryption Plain Text Message: Attack at Dawn Cipher Text Dwwdfn Dw Gdyq Cipher: Caesar Cipher Algorithm Key (3) Decryption Plain Text Message: Attack at Dawn Cipher Text Dwwdfn Dw Gdyq Cipher: Caesar Cipher Algorithm Key (3) The key can be different - How many different keys are possible?

Symmetric Algorithms - Monoalphabetic Cipher Any letter can be substituted for any other letter Each letter has to have a unique substitute There are 26! pairing of letters (~1026) Brute Force approach would be too time consuming Statistical Analysis would make it feasible to crack the key A B C D E F G H I J K L M N O P Q R S T U V W X Y Z M N B V C X Z A S D F G H J K L P O I U Y T R E W Q Statistical Analysis Knowing % of occurrences of different letters (e.g. e occurs 13% of time in the document and t occurs 19% of times) Knowing commonly occurring two and three letter combinations (e.g. in, it, the, ion, ing, …) If some knowledge about the content is available it is even easier to crack Message: Bob, I love you. Alice Cipher: Monoalphabetic Cipher Encrypted Message: Nkn, s gktc wky. mgsbc Key

Symmetric Algorithms - Polyalphabetic Cipher Developed by Blaise de Vigenere Also called Vigenere cipher Uses a sequence of monoalpabetic ciphers in tandem e.g. C1, C2, C2, C1, C2 Example Plain Text A B C D E F G H I J K L M N O P Q R S T U V W X Y Z C1(k=6) F G H I J K L M N O P Q R S T U V W X Y Z A B C D E C1(k=20) T U V W X Y Z A B C D E F G H I J K L M N O P Q R S Statistical Analysis Knowing % of occurrences of different letters (e.g. e occurs 13% of time in the document and t occurs 19% of times) Knowing commonly occurring two and three letter combinations (e.g. in, it, the, ion, ing, …) If some knowledge about the content is available it is even easier to crack Message: Bob, I love you. Alice Cipher: Monoalphabetic Cipher Encrypted Message: Gnu, n etox dhz. tenvj Key

Data Encryption Standard (DES) Goal of DES is to completely scramble the data and key so that every bit of cipher text depends on every bit of data and ever bit of key DES is a block Cipher Algorithm Encodes plaintext in 64 bit chunks One parity bit for each of the 8 bytes thus it reduces to 56 bits It is the most used algorithm Standard approved by US National Bureau of Standards for Commercial and nonclassified US government use in 1993 Cracking the DES code In 1997 it was cracked in 140 days In 1999 it was cracked in 14 hours To improve security Triple-DES has been created It uses the DES algorithm multiple times in tandem Statistical Analysis Knowing % of occurrences of different letters (e.g. e occurs 13% of time in the document and t occurs 19% of times) Knowing commonly occurring two and three letter combinations (e.g. in, it, the, ion, ing, …) If some knowledge about the content is available it is even easier to crack

Symmetric Encryption – Limitations Any exposure to the secret key compromises secrecy of ciphertext A key needs to be delivered to the recipient of the coded message for it to be deciphered Potential for eavesdropping attack during transmission of key 1. Must have done this as an assignment last year.

Asymmetric Encryption Uses a pair of keys for encryption Public key for encryption Private key for decryption Messages encoded using public key can only be decoded by the private key Secret transmission of key for decryption is not required Every entity can generate a key pair and release its public key Plain Text Cipher Cipher Text Cipher Plain Text 1. Must have done this as an assignment last year. Public Key Private Key

Asymmetric Encryption Two most popular algorithms are RSA & El Gamal RSA Developed by Ron Rivest, Adi Shamir, Len Adelman Both public and private key are interchangable El Gamal Developed by Taher ElGamal Potential for eavesdropping attack during transmission of key Efficiency is lower than Symmetric Algorithms A 1024-bit asymmetric key is equivalent to 128-bit symmetric key 1. Must have done this as an assignment last year.

Asymmetric Encryption - Weaknesses Slow compared to symmetric Encryption It is problematic to get the key pair generated for the encryption. Vulnerable to man-in-the-middle attack Hacker could generate a key pair, give the public key away and tell everybody, that it belongs to somebody else. Now, everyone believing it will use this key for encryption, resulting in the hacker being able to read the messages. If he encrypts the messages again with the public key of the real recipient, he will not be recognized easily. Cipher Trudeau’s Encrypted Message Bob Trudeau’s Message + public key David’s Public Key 1. Must have done this as an assignment last year. David’s Public Key Bob’s Message + Public key Cipher Bob’s Encrypted Message Trudeau (Middle-man) David Bob’s Public Key Trudeau’s Public Key Cipher Trudeau’s New Message + public key Cipher David’s Message + public key Trudeau’s Encrypted Message Trudeau’s Encrypted Message Attacker

Asymmetric Encryption – Session-Key Encryption Used to improve efficiency Symmetric key is used for encrypting data Asymmetric key is used for encrypting the symmetric key Plain Text Cipher (DES) Cipher Text Send to Recipient Cipher (RSA) Encrypted Key 1. Must have done this as an assignment last year. Session Key Recipient’s Public Key

Asymmetric Encryption – Encryption Protocols Pretty Good Privacy (PGP) Used to encrypt e-mail using session key encryption Combines RSA, TripleDES, and other algorithms Secure/Multipurpose Internet Mail Extension (S/MIME) Newer algorithm for securing e-mail Backed by Microsoft, RSA, AOL Secure Socket Layer(SSL) and Transport Layer Socket(TLS) Used for securing TCP/IP Traffic Mainly designed for web use Can be used for any kind of internet traffic 1. Must have done this as an assignment last year.

Asymmetric Encryption – Key Agreement Key agreement is a method to create secret key by exchanging only public keys. Example Bob sends Alice his public key Alice sends Bob her public key Bob uses Alice’s public key and his private key to generate a session key Alice uses Bob’s public key and her private key to generate a session key Using a key agreement algorithm both will generate same key Bob and Alice do not need to transfer any key 1. Must have done this as an assignment last year. Alice’s Private Key Cipher (DES) Bob’s Public Key Alice and Bob Generate Same Session Key! Bob’s Private Key Session Key Cipher (DES) Alice’s Public Key

Diffie-Hellman Mathematical Analysis Bob & Alice agree on non-secret prime p and value a Bob Alice Generate Secret Random Number x Compute Public Key ax mod p Compute Session Key (ay)x mod p Generate Secret Random Number y Compute Public Key ay mod p Compute Session Key (ax)y mod p Bob & Alice exchange public keys Identical Secret Key

Asymmetric Encryption – Key Agreement contd. Diffie-Hellman is the first key agreement algorithm Invented by Whitfield Diffie & Martin Hellman Provided ability for messages to be exchanged securely without having to have shared some information previously Inception of public key cryptography which allowed keys to be exchanged in the open No exchange of secret keys Man-in-the middle attack avoided 1. Must have done this as an assignment last year.

Authentication Authentication is the process of determining the authenticity of a message or user. Two types of authentication: Authentication of the identity presented by a remote or application participating in a session Authentication of the sender’s identity is presented along with a message. Page 120-131, Secure Electronic Commerce

Authentication – Password Based Use of secret character string only known to user and server Problems with password based authentication: Attacker learns password by social engineering Attacker cracks password by brute-force and/or guesswork Eavesdrops password if it is communicated unprotected over the network Replays an encrypted password back to the authentication server 1. Must have done this as an assignment last year.

Authentication Protocols Set of rules that governs the communication of data related to authentication between the server and the user Techniques used to build a protocol are Transformed password Password transformed using one way function before transmission Prevents eavesdropping but not replay Challenge-response Server sends a random value (challenge) to the client along with the authentication request. This must be included in the response Protects against replay Time Stamp The authentication from the client to server must have time-stamp embedded Server checks if the time is reasonable Depends on synchronization of clocks on computers One-time password New password obtained by passing user-password through one-way function n times which keeps incrementing Protects against replay as well as eavesdropping 1. Must have done this as an assignment last year.

Authentication – Kerberos 1. Must have done this as an assignment last year.

Authentication – Personal Tokens Personal Tokens are hardware devices that generate unique strings that are usually used in conjunction with passwords for authentication Different types of tokens exist Storage Token: A secret value that is stored on a token and is available after the token has been unlocked using a PIN Synchronous one-time password generator: Generate a new password periodically (e.g. each minute) based on time and a secret code stored in the token Challenge-response: Token computes a number based on a challenge value sent by the server Digital Signature Token: Contains the digital signature private key and computes a computes a digital signature on a supplied data value A variety of different physical forms of tokens exist e.g. hand-held devices, Smart Cards, PCMCIA cards, USB tokens 1. Must have done this as an assignment last year.

Authentication – Biometrics Uses certain biological characteristics for authentication Biometric reader measures physiological indicia and compares them to specified values It is not capable of securing information over the network Different techniques exist Fingerprint Recognition Voice Recognition Handwriting Recognition Face Recognition Retinal Scan Hand Geometry Recognition 1. Must have done this as an assignment last year.

Authentication – Iris Recognition The scanning process takes advantage of the natural patterns in people's irises, digitizing them for identification purposes Facts Probability of two irises producing exactly the same code: 1 in 10 to the 78th power Independent variables (degrees of freedom) extracted: 266 IrisCode record size: 512 bytes Operating systems compatibility: DOS and Windows (NT/95) Average identification speed (database of 100,000 IrisCode records): one to two seconds 1. Must have done this as an assignment last year.

Authentication – Message Digests A message digest is a fingerprint for a document Purpose of the message digest is to provide proof that a document has not been tampered with. Hash functions used to generate message digests are one way functions that have following properties It must be computationally infeasible to reverse the function It must be computationally infeasible to construct two messages which which hash to the same digest Some of the commonly used hash algorithms are MD5 – 128 bit hashing algorithm by Ron Rivest of RSA SHA & SHA-1 – 162 bit hashing algorithm developed by NIST 1. Must have done this as an assignment last year.

Authentication – Digital Signatures A digital signature is a data item which accompanies or is logically associated with a digitally encoded message. It has two goals A guarantee of the source of the data Proof that the data has not been tampered with A digital signature is created with a persons private key and verified by their public key Sender’s Private Key Sender’s Public Key Message Sent to Receiver Digest Algorithm Digest Algorithm Message Digest 1. Must have done this as an assignment last year. Same? Message Digest Signature Algorithm Digital Signature Sent to Receiver Signature Algorithm Message Digest Sender Receiver

Authentication – Digital Certificates A digital certificate is a signed statement by a trusted party that another party’s public key belongs to them. This allows one certificate authority to be authorized by a different authority (root CA) Top level certificate must be self signed Any one can start a certificate authority Name recognition is key to some one recognizing a certificate authority Verisign is industry standard certificate authority Identity Information 1. Must have done this as an assignment last year. Signature Algorithm Certificate Sender’s Public Key Certificate Authority’s Private Key

Authentication – Certificate Chaining Chaining is the practice of signing a certificate with ainother private key that has a certificate for its public key Similar to the passport having the seal of the government It is essentially a person’s public key & some identifying information signed by an authority’s private key verifying the person’s identity The authorities public key can be used to decipher the certificate The trusted party is called the certificate authority Signature Algorithm Certificate New Certificate 1. Must have done this as an assignment last year. Certificate Authority’s Private Key

Steganography & Digital Watermarfking Jagdish S. Gangolly State University of New York at Albany

Steganography vs. Cryptography Objectives: Cryptography: To hide information (its meaning) by encrypting. It enables checking for authenticity of source, non-repudiation by receiver, and protect integrity of the message in transit. Steganography: To hide information by “finger-printing” in order to prevent illicit copying of documents, or to pass private messages without detection.

Steganography I "Steganography is the art and science of communicating in a way which hides the existence of the communication. In contrast to cryptography, where the enemy is allowed to detect, intercept and modify messages without being able to violate certain security premises guaranteed by a cryptosystem, the goal of steganography is to hide messages inside other harmless messages in a way that does not allow any enemy to even detect that there is a second secret message present" [Markus Kuhn 1995-07-03].

Steganography II wax covered tablets: Herodotus’ Histories Tattoos on shaved heads Invisible inks Inks that react to chemicals Null ciphers: News Eight Weather: Tonight increasing snow. Unexpected precipitation smothers eastern towns. Be extremely cautious and use snowtires especially heading east. The highways are knowingly slippery. Highway evacuation is suspected. Police report emergency situations in downtown ending near Tuesday. Newt is upset because he thinks he is President. Source: http://www.jjtc.com/stegdoc/sec202.html

Steganography III An Example: Hide and Seek 5.0 Uses the Least Significant Bit of each pixel to encode characters, 8 pixels per character and spreads the data throughout the GIF in a somewhat randomly. Max size: 1024 x 768 pixels Maximum message size: (320x480)/8 = 19200 Message: Steganography is the art and science of communicating in a way which hides the existence of the communication. In contrast to cryptography, where the "enemy" is allowed to detect, intercept and modify messages without being able to violate certain security premises guaranteed by a cryptosystem, the goal of steganography is to hide messages inside other "harmless" messages in a way that does not allow any "enemy" to even detect that there is a second secret message present [Markus Kuhn 1995-07-03].

Steganography IV Maximum display used: 320 x 480 pixels Original: 222 x 282 pixels and 256 shades of gray Resulting image: forced to 320 x 480 pixels Source: http://www.jjtc.com/stegdoc/sec309.html

Steganography V

Steganography VI

Steganography IV Method: Each pixel is represented as a byte (8-bits). Gray-scale images are very good because the shades gradually change from byte to byte. A similar image with 16 shades of gray (four-bit color) may look very close to one with 256 shades of gray but the palette has less variations with which to work. The subtleties permit data to be stored without the human eye catching the changes. A message may be plain-text, ciphertext, other images or any thing that can be embedded in the least significant bits (LSB) of an image.